Polystyrene foams have found wide applications because of their excellent performance as a heat insulating material and a packaging and cushioning material. However, they have a low recovery of compression strain and can withstand heat at a temperature of 70.degree. to 80.degree. C. at the highest. These defects may be eliminated by using polypropylene foams or cross-linked polyethylene foams. Since, however, blowing agents used for polyolefin resins have a fast speed of dissipation, it is difficult to produce foamed particles suitable as a starting material for polyolefin foams. Any foamed particles that can be obtained has a low expansion ratio with a bulk density of 0.1 to 0.5 g/cm.sup.3 at the highest.
In an attempt to solve this problem, a process for producing high-expansion polypropylene foamed particles having a bulk density of 0.05 to 0.07 g/cm.sup.3 was proposed, which comprises dispersing polypropylene resin particles in a dispersing medium such as water in a closed vessel, maintaining the dispersion at a high pressure above the saturated vapor pressure of the dispersion and a temperature above the softening point of the polypropylene to penetrate the dispersing medium into the polypropylene resin particles, and then jetting the dispersion from the inside of the closed vessel under high pressure into the atmosphere as described in Japanese Patent Publication No. 2183/1974 which corresponds to U.S. Pat. No. 3,770,663.
According to the process of this Japanese patent document, water as the dispersing medium is utilized as a blowing agent, and high-expansion products having a bulk density of 0.016 to 0.04 g/cm.sup.3 as is the case with polystyrene foamed particles are not obtained. Attempts have been made to remedy the defect of this process by using a combination of water and a volatile organic blowing agent as a blowing agent. A process for producing polyolefin resin foamed particles which comprises dispersing polyolefin resin particles in water in a closed vessel, feeding a blowing agent into the closed vessel, heating the dispersion to a temperature above the softening point of the polyolefin resin particles but below their melting point while maintaining the pressure within the closed vessel at the vapor pressure of the blowing agent or a higher pressure, opening a discharge port provided within the sealed vessel below the water surface, and releasing the aqueous dispersion containing the polyolefin resin particles impregnated with the blowing agent into an atmosphere kept at a lower pressure than the pressure within the closed vessel has been proposed as described in Japanese Patent Application (OPI) Nos. 12035/1982, 25336/1982, 90027/1982, 195131/1982, 1732/1983, 23834/1983, 25334/1983, 33435/1983, 55231/1983, 76229/1983, 76231/1983, 76232/1983, 76233/1983, 76234/1983 and 87027/1983 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
This process can give foamed products of polypropylene resin having a bulk density of 0.026 to 0.60 g/cm.sup.3. Furthermore, it is described therein that this process can also be applied to polyethylene resins and cross-linked polyethylene resins instead of propylene copolymer resins.
A process in which in order to obtain foamed resin particles having a uniform cell diameter, a back pressure is applied by feeding an inorganic gas such as air or nitrogen gas by means of a compressor even during the releasing of the aqueous dispersion in order to maintain P.sub.1 (the pressure within the vessel after the releasing of the dispersion) at a constant value which is at least 0.7P.sub.0 in which P.sub.0 is the pressure of the dispersion before releasing from the closed vessel has been proposed as described in Japanese Patent Application (OPI) No. 55231/1983 corresponding to EP 84,803. In this process P.sub.0 is 15 to 40 kg/cm.sup.2.G. The compressor must be operated even during the releasing of the dispersion in order to feed a gas such as nitrogen gas. To maintain P.sub.1 at a pressure of 10 to 28 kg/cm.sup.2.G, for example, the compressor must have a considerably high capacity. Hence, the cost of equipment and the cost of the product become high.